The present invention relates to a process and a plant for separating air by cryogenic distillation. In particular it relates to a process using three separation columns operating at a high pressure, a low pressure and a pressure which is intermediate between the high and low pressures.
It is known from EP-A-0538118 to use a process of this type in order to separate air, the intermediate-pressure column having a bottom reboiler heated by nitrogen from the high-pressure column, thus reducing the heating of the bottom reboiler from the low-pressure column.
One aim of the invention is to reduce the energy consumption of the separation process with respect to the processes of the prior art.
Another aim of the invention is to produce oxygen with a purity of at least 95 mol %, or even at least 98 mol %, with an improved yield.
FIG. 1 shows a conventional process with a low-pressure column 103 operating at 1.3 bara enabling oxygen to be produced at 99.5 mol % with a yield of 92%.
A stream of 1 000 Nm3/h of air 1 at about 5 bara is divided into two in order to form a first stream 17 and a second stream 3 which is supercharged in a super-charger 5 at a higher pressure of about 75 bara.
The two streams 3, 17 are cooled on passing through a heat exchanger 100. The stream 17 is sent to the bottom of the high-pressure column 101 and the liquefied stream 3 in the heat exchanger 100 is expanded in a turbine 6 producing an at least partially liquid stream at its outlet, the fluid or mixture of fluids leaving the turbine 6 being sent at least in part to the high-pressure column 101.
A rich liquid stream 10 from the high-pressure column 101 is cooled in the subcooler 83 before being expanded and sent to an intermediate level of the low-pressure column 103.
A liquid airstream 12 is withdrawn from the high-pressure column 101, cooled in the subcooler 83, expanded and sent to the low-pressure column 103.
A waste nitrogen stream 72 is withdrawn from the top of the low-pressure column 103, sent to the subcooler 83 and then to the heat exchanger 100 where it is warmed.
A stream 31 of 193 Nm3/h of oxygen at 99.5 mol % is withdrawn in liquid form from the low-pressure column 103, pumped in the pump 19 to 40 bara and vaporized in the heat exchanger 100 in order to form a pressurized gas stream.
A stream of 200 Nm3/h of gaseous nitrogen 33 is withdrawn from the top of the high-pressure column 101 and is partially heated in the heat exchanger 100. At an intermediate temperature, part of the gas is expanded in a turbine 35 before being mixed with the waste gas 72.
In another conventional diagram illustrated in FIG. 2, the low-pressure column operates at 4.8 bara and the high-pressure column 101 operates at 14.3 bara. This process produces oxygen at 99.5 mol % with a yield of 78%.
A flow of 1 000 Nm3/h of air 1 at about 14.3 bara is divided into two in order to form a first stream 17 and a second stream 3 which is supercharged in a super-charger 5 to a higher pressure of about 75 bara.
The two streams 3, 17 are cooled on passing through a heat exchanger 100. The stream 17 is sent to the bottom of the high-pressure column 101 and the liquid stream 3 is expanded in a turbine 6 producing an at least partially liquid stream at its outlet, the fluid or mixture of fluids leaving the turbine 6 being sent at least in part to the high-pressure column 101.
A rich liquid stream 10 from the high-pressure column 101 is cooled in the subcooler 83 before being expanded and sent to an intermediate level of the low-pressure column 103.
A liquid airstream 12 is withdrawn from the high-pressure column 101, cooled in the subcooler 83, expanded and sent to the low-pressure column 103.
A waste nitrogen stream 72 is withdrawn from the top of the low-pressure column 103, sent to the subcooler 83 and then to the heat exchanger 100 where it is warmed.
A stream 31 of 164 Nm3/h of oxygen at 99.5 mol % is withdrawn in liquid form from the low-pressure column, pumped in the pump 19 to 40 bara and vaporized in the heat exchanger 100 in order to form a pressurized gas stream.
No gaseous nitrogen stream is withdrawn from the top of the high-pressure column 101 (of course a high-pressure gaseous nitrogen stream is condensed conventionally in a reboiler-condenser associated with the low-pressure column).
It is known from EP-A-833118 and U.S. Pat. No. 5,657,644 to heat an intermediate-pressure column of a triple-column system with an argon-enriched gas which also serves to feed an argon-production column.
The inventors of the present application have discovered that, even without using an argon-separation column, purification of the oxygen at the bottom of the low-pressure column remains satisfactory for the production of high-purity oxygen.